An intermediate shaft of a vehicle steering apparatus is configured of a telescopic shaft in which, for example, a male shaft and a cylindrical female shaft are telescopically connected in an axial direction by splines in mesh (spline fitting). In some cases, the outer circumferential surface of the male shaft or the inner circumferential surface of the female shaft, which has splines, is coated with a resin coating layer with which a clearance between both the shafts is filled so as to reduce rattling or to reduce loosening of a steering wheel during a steering operation (for example, see PTLs 1 and 2).
As a method for forming the resin coating layer, a powder-fluidized bed dip method is known. Since an organic solvent is not used in the powder-fluidized bed dip method, it is advantageous to reduce the impact on the environment.
In the powder-fluidized bed dip method, a powder coating material containing a base resin (thermoplastic resin) as a base of the resin coating layer is first prepared, and the powder coating material enters a state of being suspended or fluidized in a fluid bath by blowing air or the like therein.
Subsequently, when the male shaft or the female shaft heated to have a temperature equal to or above the melting point of the base resin is dipped in the suspended or fluidized powder coating material, the powder coating material is attached to the outer circumferential surface of the male shaft or the inner circumferential surface of the female shaft (hereinafter, in some cases, also referred to as an “attachment target surface”) and is melted and spread, and then, is further cooled and solidified such that a resin coating layer is formed.
As the base resin as the base of the resin coating layer formed from the powder coating material, polyamide 11, polyamide 12, or the like, which can form a resin coating layer good in fluidity when melting and good in slidability or the like, is appropriately used.